Not Applicable
1. Field of the Invention
This invention relates to optical testing of the eye""s sensitivity to light, and in particular to visual field evaluation, using a Virtual Reality system.
2. Background Art
In the field of medicine where disorders of the eye are treated, it is necessary to measure the sensitivity to light in various regions of the light-sensitive retina. So doing measures function, as well as quantifying disorders of the eye and the retina, the optic nerve, the optic chiasm, the visual pathways to the brain, and the brain itself. Visual field testing is mandatory for glaucoma diagnosis and treatment. Apparatus to measure the field of vision is used by ophthalmologists and optometrists for these purposes and is relatively complex in its various functions, some of which complexity tends to make the human patient become tired or lose attention to the test.
One particular, currently known, visual field testing strategy employs image patterns created by a technology called xe2x80x9cflicker perimetry.xe2x80x9d In this method, high temporal frequencies of flicker stimulate, on a preferential basis, retinal ganglion cells projecting onto the magnocellular layers of the lateral geniculate body of the brain. Such projections of ganglion cells are called M-cell fibers, and these fibers primarily consist of large-diameter cell axons. Comprising only 3-5 per cent of all retinal ganglion cells, it is these large-diameter M-cell axons which are particularly susceptible to glaucomatous damage. As these M-cells are stimulated preferentially by flicker perimetry, this test has promise for determining early stages of glaucoma in patients who are glaucoma suspects.
Employing flicker perimetric visual testing strategies, localized flicker field deficiencies have been reported in patients who have conventional light-sense visual fields interpreted as normal. Flicker perimetry, then, by diagnosing damage to M-cells fibers before more generalized cellular damage occurs, may well have clinical value in differentiating ocular hypertension from primary-open angle glaucoma.
Additionally, it has been noted that flicker perimetry has the distinct advantage of being more resistant to blur, scattering, and image degradation than conventional light-sense automated perimetry. For patients with significant cataracts, the test light target in light-sense perimetry is often poorly visualized, resulting in visual fields of dubious quality. Since media opacification (typically cataracts) does not unduly influence flicker perimetry, the use of flicker perimetry can be quite useful for such patients. For older persons, then, this relative resistance to image degradation is an especially important advantage of flicker perimetry, as cataracts and subsequent visual degradation are far more prevalent in older persons.
An additional advantage of flicker perimetry related to its blur resistance of up to six diopters is that refractive lenses or glasses for distance or for near are generally not required to compensate for the patient""s refractive error or for accommodation.
It is known to employ a method for flicker perimetry which determines the highest frequency of flicker (called the critical flicker frequency) which can be detected for a 100 percent contrast flicker target. Another known flicker perimetric strategy determines the minimum contrast required to detect flicker for a fixed temporal frequency or group of frequencies. This strategy uses primarily temporal frequencies of 2, 8, and 16 Hz.
At an even higher frequency, sinusoidal gradings at 25 Hz are employed in the table model Humphrey/Welch Allen xe2x80x9cHumphrey FDT(trademark) Visual Field Instrument.xe2x80x9d A phenomenon called xe2x80x9cfrequency doublingxe2x80x9d is called into play with this instrument. Frequency doubling technology perimetry creates an illusion in which a low-spatial frequency sinusoidal grading, (less than 1 cycle per degree) undergoes high-temporal-frequency counterphase flicker (greater than 15 Hz). The stimulus then appears perceptually to have twice as many light and dark bars as are actually physically present. This illusion is mediated neurologically by the M-cells, described above, which project onto the magnocellular layers of the lateral geniculate body. Cello et al. noted in the American Journal of Ophthalmology that frequency-doubling perimetry xe2x80x9cdemonstrates high sensitivity and specificity for detection of early, moderate, and advanced glaucomatous field loss.xe2x80x9d Cello adds that frequency-doubling perimetry xe2x80x9cprovides a useful complement to conventional automated perimetry test procedures and can serve as an effective initial visual field evaluation for detection of glaucomatous visual field loss.xe2x80x9d
A new visual field testing strategy, called the Swedish interactive test algorithm (SITA(trademark)), has been introduced by Humphrey Systems for its light-sense automated perimetric system. This testing strategy is said to reduce the threshold time for visual field performance on the Humphrey visual field tester by approximately 50 percent, while preserving the same reliability. Cello et al. postulate that xe2x80x9csimilar methods could be applied to the threshold strategies for frequency-doubling technology perimetry,xe2x80x9d adding that xe2x80x9cit is conceivable that a frequency-doubling technology perimetry threshold could be employed to reduce the testing time to approximately 2.0 to 2.5 minutes per eye, with test-retest reliability equivalent to that of current threshold methods.xe2x80x9d With this in mind, the present invention envisions the incorporation of new algorithms, such as described above, to reduce test time and enhance patient friendliness.
Two of the present inventors disclosed in U.S. Pat. No. 5,898,474, issued Apr. 27, 1999, a method and apparatus for using virtual reality principles for testing and quantifying visual information from the eye, the visual pathways, and the brain. A headgear configuration allows the patient to observe a field of view into which sequenced test stimulae are presented by an excitation device commanded by a computer. Interactive sensory feedback both to and from the patient enables computer-driven presentation and modulation of test stimuli to measure with precision such parameters as visual field performance, visual acuity, and color vision. Using this system allows the patient unprecedented freedom of movement of the head and body, thus minimizing or even eliminating the stress and fatigue common with conventional non-virtual-reality visual field testing systems.
The purpose of the presently described method and apparatus for visual field testing is to allow the sensitivity of the visual field to be measured without the attendant stress of the patient, and yet to preserve accuracy. The means by which this is accomplished uses concepts and apparatus from Virtual Reality. Virtual Reality is a term applied loosely to the experience of an individual when exposed to the appearance of surroundings which are presented by interactive apparatus for stimulation of the senses. The primary cues are usually visual, supplemented by audio, and the feedback to the apparatus is generally by physical movements of the individual experiencing the Virtual Reality, such as pressing a button or a switch, or speaking into a microphone.
In the parent patent applications and the presently disclosed invention, a Virtual Reality visual field measuring method and a related apparatus use a head-mounted goggle or face mask unit to present visual and audio stimuli to a patient. The visual portion has both relatively fixed image information, and superimposed visual areas, which may vary in time, place, color, and intensity. These stimuli are generated and controlled by software in an associated computer, which receives interactive feedback stimuli from the patient. Such stimuli include, but are not limited to, direction of gaze sensing, eyelid movement and blinking, audio, and hand pressure signals on cue.
Content of the software is dictated by the need to provide technically acceptable protocols. Such protocols provide for examining wide and narrow fields of view, selected areas, such as the blind spot of the fovea, and measurements of thresholds for sensitivity to light intensity, or, if desired, color. These are usually done for one eye at a time, each looking at the same, or similar, field of views.
Active feedback sensing alerts the system to patient loss of attention in general, or loss of fixation in particular, for notation and reiteration of test stimuli. In the presently described method and apparatus, provision is also made for reiteration of individual test points when a result is found to be inconsistent with a predetermined norm, or when lack of concentration or poor cooperation becomes evident, with appropriate care taken to provide no leading cues which may cause false positive or false negative responses. The software allows optional restful imagery to be provided in the xe2x80x9cbackground,xe2x80x9d in addition to a conventional, uniform featureless field. The imagery in various quadrants/areas may be patterns, or low-contract images, and may move quickly or slowly, and may have intensity, color, or temporal modulation. The intensity, color, location, and duration of the superimposed test points are displayed by conventional electronic means, such as are now used in image presentations. Such means include cathode-ray tube, electroluminescent, liquid crystal, and gas discharge panels. Alternatively, test information may be projected onto the retina by means of one, or more, virtual retinal display systems. A hard-copy printout documenting patient responses is provided for the physician""s records.
The present invention employs flicker perimetry and a subset of flicker perimetry, frequency-doubling technology perimetry, in a head-mounted display visual field testing system. The head-mounted display element of the present invention can be in the form of goggles or other head-gear configuration, all of which allow unprecedented freedom of motion for the patient when compared to stationary visual-field testers, which preclude neck or head motion. Because refractive lenses or glasses for distance or for near are generally not required to compensate for the patient""s refractive error or for accommodation, the head-mounted display of the present invention is less complex than one requiring compensation for the patient""s refractive error and accommodation, and, consequently, is also more economical to manufacture. The present invention of utilizing flicker perimetry and frequency-doubling technology perimetry for visual field testing in a virtual reality head-mounted display setting is a novel advance in visual field/function testing. The excitation device providing the flicker signal and the frequency-doubling technology signal include projection systems whereby the signal is projected onto the display screen of the head-mounted display, or, alternatively, one or more virtual retinal display projection systems projecting the flicker signal and the frequency-doubling technology signal directly onto the patient""s retina may be employed.
The flicker signal and the frequency-doubling technology signal can be varied in intensity, duration, color (such as, but not limited to, blue-on-yellow, or yellow-on-blue), frequency, size, and locus within the patient""s visual field.
An object of the present system is to provide relief from the stress of being required to concentrate, without head movement, one""s gaze at a fixed location. The gaze sensor may be multi-element, so as to allow the gaze to be detected in a small solid angular range and, within this range, the effective fixation will be deemed to be maintained. The software may include an interest-fixation icon which encourages the gaze to trace its motion within the allowed solid angle, thus avoiding fixation fatigue. The software keeps track of the location of the test point frame of reference within that solid angle of displacement, so as to provide accurate mapping of test data on the field of view presented to the retina.
In addition to visual field testing, it is certainly within the scope of this invention to employ flicker perimetry and frequency doubling technology perimetry to provide other Virtual Reality computer-driven, interactive testing capability, such as for visual acuity and color testing.
The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, in which similar reference characters refer to similar parts, and in which: